Extracting hydrocarbons, like oil and gas, from geologic formations within the earth is an expensive operation. Before committing the required substantial costs necessary to extract hydrocarbons from a given formation, an operator must evaluate the potential of the formation to produce hydrocarbons. In other words, the operator wants to know how much hydrocarbon they may recover and how difficult will it be to recover it.
Source rock is rock that is rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Clearly, an operator would like to know the amount of organic matter, or the “organic richness” of the rock in a formation to evaluate whether to commit the resources necessary to recover the rock.
Accurately quantifying organic richness of source rock is difficult. The properties and concentrations in source rocks can vary at small vertical intervals (tenths of millimeters). Evaluation therefore requires high measurement and sample resolution. Typically, assessment requires recovering rock samples and performing laboratory analysis on them to determine parameters such as TOC, mineralogy, vitrinite reflectance and ratios of carbon/oxygen and carbon/hydrogen.
It would be easier and less expensive to assess organic richness without physically recovering the rock by using standard well logging techniques, such as density, neutron, gamma ray, acoustic, and/or formation resistivity. However, those techniques lack the resolution necessary to yield meaningful information in many situations.
It has previously been shown that overlaying a properly scaled sonic transit time log on a resistivity curve (determined from a deep-reading resistivity logging tool) can yield information relating to organic richness. That technique, referred to as Δ log R technique, was described by Passey et al., (A Practical Model for Organic Richness from Porosity and Resistivity Logs, Am. Ass. Petr. Geo. Bull., V. 74, No. 12, 1990, 1777-94). In water-saturated, organic-lean rocks, the resistivity log and the porosity log parallel each other because both measurements respond similarly to variations in formation porosity. In organic-rich rock, the resistivity log and the porosity log respond opposite to each other, yielding a divergence between the overlaid logs. The divergence between the porosity log and the resistivity log can thus be used to indicate organic-rich formations.
While the Passey Δ log R technique provides some information relating to the organic richness of formation rock, the technique lacks spatial resolution because the resistivity log is collected at a single depth point, which depending on the sample rate and bed thickness can result in an averaging affect combining thin adjacent contrasting beds. Thus, there remains a need for higher-resolution and oriented logging techniques capable of characterizing the detailed distribution of organic matter and orientation of source rock formations and other unconventional hydrocarbon plays.